Gary, I have had issues with some of my Thunder Tiger PRO 40's doing this same thing. In my case, it was the tapered drive washer sliding down the crankshaft bolt and not locking tight. What I did was take a thin strip of .014 Mylar sheet and place it under the tapered nut before tightening the prop down. I then wiggled the prop back and forth and tugged on the Mylar till it cam out. It took a couple of times, but I eventually got it.
Another method is to tighten the prop nut and prop down good, and give the prop nut a solid tap with a hammer. this should knock the crankshaft back some, but still leave it tight. Be sure to use a prop nut you don't care about if you try this, because it will end up with a flat spot on the tip. I am sure you realize this could potentially damage the engine, but I know you are smart enough to know that!
Best of luck...